A typical power supply circuit receives power from an AC source, such as for example a conventional mains input. During a normal mode of operation, the power supply circuit generates either a constant voltage or a constant current to a DC load. A first-stage power factor correction (PFC) circuit is known to maintain the current drawn from the AC source substantially in phase with the voltage of the AC source. The PFC circuit provides a high power factor and a stiff first-stage output voltage to a second-stage DC-to-DC converter circuit. The second-stage DC-to-DC converter circuit regulates the constant voltage or the constant current for the DC load to meet an output performance requirement.
The PFC circuit may be based on various topologies. One such topology is a boost converter. Another such topology is a flyback converter. Although both types of converters can be used in a PFC circuit, a PFC converter based on a boost converter topology typically can achieve a greater power factor (e.g., closer to 1) than a PFC converter based on a flyback topology.
PFC circuits are typically coupled across an output rectifier filter capacitor between the output terminals of a rectifier and are configured to receive DC voltage. When the DC load coupled to the PFC circuit is heavy, the PFC circuit may function optimally in a critical conduction mode. When the DC load coupled to the PFC circuit is light, the PFC circuit may function in a pulsing mode. The pulsing mode is generally due to an inability of the output rectifier filter capacitor being able to fully discharge during an off time of an electronic switch of the PFC circuit. Unfortunately, in LED driver applications the pulsing mode is highly undesirable because an LED load is very sensitive to variations in the voltage, and as a result the LEDs might flicker.